Increasing evidence suggests that both fasting and overfeeding affect mammalian thermogenesis; fasting decreases and overfeeding increases an animal's heat-production (oxygen consumption) in the resting state. Since these dietary manipulations produce parallel changes in the activity of sympathetic nerves, a system of recognized thermogenic importance, and since various animal models of obesity display abnormalities in thermogenesis, the current research proposal seeks to examine different obese animals for evidence of defective sympathetic regulation particularly in response to changes in diet or in environmental temperature. During the past year the technique utilized by the applicant for assessing sympathetic activity, the norepinephrine turnover technique, has been applied to the study of brown adipose tissue, the principal thermogenic tissue in rats and mice. Preliminary results indicate that fasting decreases sympathetic activity in brown fat and that chronic cold exposure and overfeeding both lead to tissue hypertrophy and to increased sympathetic activity in brown fat. Similar measurements of sympathetic activity in brown fat will be made in the various animal models of obesity under study.